In a typical cruise speed control (CSC) system of a modern advanced light jet aircraft, a full authority digital engine control (FADEC) unit (also referred to as an engine control unit (ECU)) modulates the engine fuel flow, by controlling the throttle position, to substantially maintain the flight speed constant. A challenge of the CSC system is that it has to operate within many boundaries as opposed to a full authority auto-throttle system that can be found in modern higher end aircraft. The CSC system is generally designed to operate in conjunction with an engine synchronization (SYNC) function, which has a very limited authority band relative to the overall authority of the actuation, in this case, engine thrust.
For example, the SYNC function can have an authority deviation of a few percent of the engine speed (N1) command. On the other hand, the SYNC function may not be available in the full cockpit throttle range, but rather, in a limited throttle/thrust lever angle (TLA). In addition, because each engine can be designed to maintain their independence even during the SYNC function operation and the cockpit controls of each engine are independent, the reference point for the command limiting in each engine can be different (asymmetric in case of a twin engine aircraft). And when another function, like the CSC system function is added to the engine control, it can have its own safety and/or authority limits. When all these different system/function limits are globally considered and when each of the limits has to be continuously obeyed during operation, a complex scenario can occur as to how to simultaneously coordinate and operate within the differing limits.